[GraphHopper] Disconnected areas

[Peter K]

Hi Ludwig,

> Thanks for the reply.
>
> Just so that I understand the trade-off: basically, the intention is
> to exclude areas that would not allow a meaningful routing anyway
> (apart from its own subnetwork) and the price to lower this value is a
> size question for the resulting routing files?

The difference of the size is negligible. It is just about meaningful
routing: with several subareas (and there are many!!) the location
lookup ends up more often in that areas and would lead to "no route
found - disconnected areas" errors. E.g. just clicking into the map or
dragging the flags would result a lot more into that errors.

To really fix this problem one would have to find out if it is an island
with water around (then it should be routable regardless of the number
of nodes) or if it is a part of a city or other terrain (like
forestry/private streets/...).

No easy idea how to do that, but the configurable option now could make
everyone happy.

> I am not really familiar with the routing algorithm, I am just amazed
> that it works so well and so fast!

Thanks! ... and it is just the beginning ;)
Especially on Android there are a lot more things to do!

Regards,
Peter.

>
>
>
>
> On 24 September 2013 15:45, Peter K <peathal at yahoo.de
> <mailto:peathal at yahoo.de>> wrote:
>
>     Hi Ludwig,
>
>     > Hi Peter,
>     >
>     > fiddling with that parameter seems to do the trick. I managed to get
>     > routing to work for Naxos with a value of 500, but had to drop this
>     > further to 200 to get routing on some even smaller islands.
>
>     Good and bad ;(!
>
>     > For me at this point the question is what the motivation is to
>     exlude
>     > certain areas at all. Even if this was a private area (behind a
>     > barrier) why exclude it from roiuting?
>
>     Because you won't find a route if one coordinate of the route is
>     inside
>     and one is outside. E.g. if you would search 'Dresden' (or a lot more
>     locations like 'moskau' etc) the GPS coordinate would be exactly
>     within
>     such an area and you won't find a route. Also routing within
>     private/restricted/forestry areas is in my opinion not really
>     useful for
>     a public site. Additionally there are OSM bugs which leads to those
>     subnetworks.
>
>     Do you have another idea? One could try to solve this via a changed
>     routing or location-lookup but this looks ugly to me.
>
>     I've lowered this value to 200 and made it configurable via
>     prepare.minNetworkSize:
>     https://github.com/graphhopper/graphhopper/issues/86
>
>     Regards,
>     Peter.
>

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